1. Field of the Invention
The present invention relates to an improved process for catalytic conversion of the H.sub.2 S and SO.sub.2 present in a gaseous stream to elemental sulfur and the subsequent removal of the sulfur. More specifically, it is concerned with the removal of sulfur from the tail gas of a conventional Claus plant by the use of a Cold Bed Adsorption process. This improved process is particularly useful for design of new plants to obtain higher conversion to elemental sulfur than was formerly required. It can also be used to bring an already existing Claus plant involving a series of Claus reactors into compliance with the contemporary restrictive SO.sub.2 emission standards.
2. Description of the Prior Art
In the so-called Cold Bed Adsorption (CBA) process, typically as described in U.S. Pat. Nos. 3,702,844, 3,758,676, and 3,749,762, the hydrogen sulfide and sulfur dioxide content of Claus plant gas streams is decreased by conversion to elemental sulfur in the presence of a Claus-type catalyst at a temperature between about 270.degree. and 300.degree. F. Since the Claus reaction is a reversible exothermic reaction and since the chemical process that occurs in the reactor can be viewed as an approach to chemical equilibrium, the lower temperatures associated with the CBA reactor have in principle two advantages over the higher temperature Claus reactor, each of which contributes to lower reactant concentrations and more efficient removal of sulfur. Namely, the temperature dependence of the thermodynamic equilibrium constant of an exothermic reaction favors lower reactant concentrations as the temperature decreases. And, the particular temperature range of the CBA reactor is below the dew point of sulfur, thus physical deposition of the reaction product (sulfur) as an adsorbed phase occurs. This deposition reduces the concentration of the sulfur in the gaseous reaction mixture and thus further reduces the equilibrium concentrations of the reactants (H.sub.2 S and SO.sub.2). However, this is not an advantage in that the sulfur deposited on the catalyst bed will ultimately prevent the catalytic reaction from occurring. Thus, the CBA reactor, unlike the high-temperature Claus reactor, must periodically be regenerated by vaporizing the deposited sulfur with a hot stripping gas followed by a cooling back to the desired operating temperature.
Various methods of accomplishing the overall CBA process, including the required regeneration and cooling of the catalyst bed, have been proposed. For example, in U.S. Pat. No. 3,702,884, a pair of CBA units is used as an addition to a Claus plant wherein one of the reactors is operated in the cleanup mode, while the other is being regenerated and cooled off-stream in a recycle mode. Usually, the off-stream regeneration involves a condenser, blower, and heater to recycle the gas already present in the CBA unit with makeup gas being added only to compensate for volume changes associated with the cooling of the gas. A large amount of fuel gas is consumed in the heater to heat the recycle gas to the relatively high temperature required for regeneration. In U.S. Pat. Nos. 3,758,676 and 3,749,762, more complicated processes involving at least three catalytic reactors are disclosed wherein the high temperature level of the stripping gas, which is required for regeneration, is achieved by Claus reaction in the regeneration reactor, and all reactors are continuously on-stream acting in either a Claus clean up, CBA clean up, or CBA regeneration and cooling mode. In both of these processes the regeneration and cooling steps are accomplished by altering the relative positions of the reactors in the overall flow as well as controlling the temperature, but all reactors stay on-stream. There is no recycle off-stream as was the case in U.S. Pat. No. 3,702,884. All three processes, when properly designed and engineered, can achieve as high as 99 percent removal of sulfur. However, by attempting to maintain these high conversion levels, the sulfur losses due to COS, CS.sub.2 and elemental sulfur play a more significant role. In particular, the hydrolysis of the COS and CS.sub.2 becomes a critical consideration and a primary objective. For a more detailed discussion of their role, see a paper presented at the Gas Processors Association 53rd Annual Convention, Mar. 25 through 27, 1974, Denver, Colo., authored by C. S. Goddin, E. B. Hunt, and J. W. Palm, and entitled "Amoco CBA Process for Improving Claus Plant Recovery," herein incorporated by reference.
When attempting to achieve and maintain high conversion levels from an existing Claus plant by the addition of a CBA reactor downstream, various pragmatic considerations complicate the situation. For example, if the presently existing Claus plant is already being operated at maximum capacity, it may be necessary to maintain the space velocities and gross capacity of the Claus portion at its previous level at all times to satisfy the needs of the particular plant. Yet simultaneously, the concept of utilizing an indigenous stream of this Claus plant as a source stream to regenerate the CBA reactors may be economically mandatory but inconsistent with maintaining gross capacity. In view of these problems, we have discovered a particular method of implementing a CBA process downstream from an existing Claus plant which essentially preserves the capacity of the Claus plant, allows for optimization of the hydrolysis of the COS and CS.sub.2, thus maintaining high conversion levels and still has the economic benefits of using an indigenous stream to accomplish the regeneration of the sulfur-fouled CBA reactors. Such a method can also be used advantageously in designing new plants, including situations where conventional Claus reactors are installed initially with the CBA reactors being added later when needed.